Photographic emulsions rely on silver halide grains for light sensitivity. The light sensitive silver halide grains exhibit a face centered cubic crystal lattice structure that typically forms {111} or {100} crystal faces. Silver chloride and silver bromide both form face centered cubic crystal lattice structures. Photographically useful grains can consist of silver chloride, silver bromide and any combination of these two silver halides. Silver iodide under the grain precipitation conditions employed for preparing photographic emulsions does not form a face centered cubic crystal lattice structure and hence does not form grains having {111} or {100} crystal faces. Silver iodide nevertheless can be accommodated in minor proportions in grains having {111} or {100} crystal faces. In grains composed of two or more halides the halides are named in their order of ascending concentrations.
Maskasky U.S. Pat. Nos. 4,435,501, 4,463,087 and 4,471,050 demonstrated that the epitaxial deposition of a silver salt onto the corners or edges of host silver halide grains can produce emulsions of increased sensitivity. In Maskasky '501 and '087 silver halides forming face centered cubic crystal lattice structures and hence isomorphic (x, y and z unit cell axes of equal length) silver halides are disclosed to be directed onto the edges and/or corners of host grains having {111} crystal faces by relying on one or a combination of (a) bulk iodide in the host grain, (b) adsorbed iodide on the surface of the host grain, and (c) adsorbed dye capable of acting as a deposition site director. In Maskasky '050 it was observed that nonisomorphic (lacking a face centered cubic crystal lattice structure) silver salts can be deposited onto the edges and corners of host grains having {111} or {100} crystal faces even in the absence a site director.
Chen et al EPO 0 498 302 A1 discloses preparing emulsions with protrusions having a higher solubility than the host grains to improve developability. The protrusions can be distributed over the grain faces or, by using one of the types of site directors taught by Maskasky, cited above, can be directed to the edges or corners of the grains. In the examples the host grains are stated to be octahedral. That is, they are regular grains having eight {111} crystal faces.
Maskasky U.S. Pat. No. 5,275,930 discloses the formation of high (&gt;50 mole %) chloride tabular grains having {100} major faces which are increased in sensitivity by depositing at one or more of their corners a silver halide that contains less than 75 percent of the chloride ion concentration of the host grains. The purpose of limiting the chloride concentration of the epitaxy is to decrease its solubility in relation to the solubility of the host grains. This insures that the epitaxy is confined to the corners of the host grains rather than spreading over the host grain surface, thereby dissipating the increase in sensitivity being sought. Even with the lower solubility halide composition of the epitaxy Maskasky suggests employing morphological stabilizers, including compounds such as benzothiazole, to minimize spreading of the epitaxy over the host grain surfaces in the heating step that occurs during chemical sensitization.